This invention relates to implantable cardiac stimulating devices and particularly to implantable cardiac pacemakers which use implantable leads. More particularly, this invention relates to a system and method for providing hemodynamically optimal pacing therapy to a patient while the patient is at rest and for providing hemodynamically optimal rate-responsive pacing therapy.
A pacemaker is an implantable medical device which delivers electrical stimulation pulses to cardiac tissue to relieve symptoms associated with bradycardia--a condition in which a patient cannot normally maintain a physiologically acceptable heart rate. Early pacemakers delivered stimulation pulses at regular intervals in order to maintain a predetermined heart rate--typically a rate deemed to be appropriate for the patient at rest. The predetermined rate was usually set at the time the pacemaker was implanted, although in more advanced pacemakers, the rate could be set remotely after implantation.
Early advances in pacemaker technology included the ability to sense the patient's natural cardiac rhythm (i.e., the patient's intracardiac electrogram, or "IEGM"). This led to the development of "demand pacemakers"--so named because they deliver stimulation pulses only as needed by the heart. Demand pacemakers are capable of detecting a spontaneous, cardiac depolarization which occurs within a predetermined time period (commonly referred to as the "escape interval") following a preceding depolarization. When a naturally occurring depolarization is detected within the escape interval, the demand pacemaker does not deliver a pacing pulse. The ability of demand pacemakers to avoid delivery of unnecessary stimulation pulses is desirable because by doing so, battery life is extended. Furthermore, stimulation in the heart's vulnerable period during the T-wave is avoided, which is otherwise known to cause fibrillation.
Modern pacemakers allow physicians to telemetrically adjust the length of the escape interval, which has the effect of altering the heart rate maintained by the device. However, in early devices, this flexibility only allowed for adjustments to a fixed programmed rate, and did not accommodate patients who required increased or decreased heart rates to meet changing physiological requirements during periods of elevated or reduced physical activity. Therefore, unlike a person with a properly functioning heart, a patient receiving therapy from an early demand pacemaker was paced at a constant heart rate--regardless of the level to which the patient was engaged in physical activity. Thus, during periods of elevated physical activity, the patient was subject to adverse physiological consequences, including lightheadedness and episodes of fainting, because the heart rate was forced by the pacemaker to remain constant.
The adverse effects of constant rate pacing lead to the development of "rate-responsive pacemakers" which can automatically adjust the patient's heart rate in accordance with metabolic demands. An implanted rate-responsive pacemaker typically operates to maintain a predetermined minimum heart rate when the patient's level of metabolic need is at or below a threshold level, and gradually increases the maintained heart rate in accordance with increases in metabolic need until a maximum rate is reached. Rate-responsive pacemakers typically include processing and control circuitry that correlates a physiological parameter indicative of metabolic need to a desirable heart rate. In many rate-responsive pacemakers, the minimum heart rate, maximum heart rate, and the transition rates between the minimum heart rate and the maximum heart rate are parameters that may be adjusted to meet the needs of a particular patient.
One approach that has been considered for enabling rate-responsive pacemakers to determine an appropriate heart rate involves measuring a physiological parameter that reflects the level to which the patient is engaged in physical activity. Physiological parameters that have been considered include central venous blood temperature, blood pH level, QT time interval and respiration rate. However, certain drawbacks (such as slow response time, unpredictable emotionally-induced variations, and wide variability across individuals) render the use of these physiological parameters difficult, and accordingly, they have not been widely used in practice.
Rather, most rate-responsive pacemakers employ sensors that transduce mechanical forces associated with physical activity. These activity sensors generally contain a piezoelectric transducing element which generates a measurable electrical potential when a mechanical stress resulting from physical activity is experienced by the sensor. By analyzing the signal from a piezoelectric activity sensor, a rate-responsive pacemaker can determine how frequently pacing pulses should be applied to the patient's heart.
Some rate-responsive pacemakers monitor physiologic parameters, other than the IEGM, which reflect hemodynamic performance. For example, U.S. Pat. No. 4,774,950 of Cohen refers to a system that measures mean pressure at a variety of locations (e.g., mean arterial pressure, mean right ventricular pressure, mean left atrial pressure, mean left ventricular pressure or mean central venous pressure). For a selected mean pressure, a short term current mean pressure is compared to a long term mean baseline pressure. The mean pressure data may be used as a hemodynamic indicator to infer stroke volume by taking pressure-time histories of arterial blood flow. The inferred stroke volume may be used to control the rate of pacing.
The use of another hemodynamic indicator in rate-responsive pacing, blood oxygen level, is described in U.S. Pat. No. 4,967,748 of Cohen. Blood oxygen level is measured at a particular site in the circulatory system of a patient. The blood oxygen level measurements may be used to control the rate of pacing by altering the stimulating frequency of an associated pacemaker.
Another example of using pressure as a hemodynamic indicator in rate-responsive pacing is described in U.S. Pat. No. 4,708,143 of Schroeppel. That patent describes the use of a pressure sensor to sense the opening and closure of the tricuspid valve, the sensed opening and closure being used to determine ejection time. The changes in ejection time are processed to determine a corresponding change in stroke volume that is used to adjust the pacing rate of a pacemaker.
It is known that during periods of increased metabolic need, cardiac output may be optimized by adjusting various parameters in a rate-responsive pacemaker, such as heart rate, because cardiac output is the product of heart rate and stroke volume. The above-identified U.S. Pat. No. 4,708,143 describes a method where changes in stroke volume are determined from a pressure sensor and applied to a look-up table to determine a corresponding change in cardiac rate to achieve a predetermined level of cardiac output. The new cardiac rate replaces the previous rate. In this manner, the physician may set the pacemaker to an initial heart rate and the pacemaker may attempt to optimize, or at least improve, cardiac performance by adjusting the heart rate in response to changes in stroke volume, in accordance with a predetermined relationship (i.e., the look-up table).
Another patent describing stroke volume controlled rate-responsive pacing is U.S. Pat. No. 4,535,774 of Olson. Olson describes a rate-responsive pacemaker which paces the heart at a rate dependent on detected variations in the stroke volume of the heart. Olson infers stroke volume by placing an electrode system into the heart and injecting a current between these electrodes and measuring the voltage between the electrodes. Changes in the measured voltage are related to the changes in impedance of the heart cavity, with impedance changes being related to stroke volume. The inferred stroke volume is compared to a reference stroke volume, with the difference being used to compute a corresponding change in heart rate. Once again, the physician sets an initial heart rate and the pacemaker adjusts the rate to attempt to provide rate-responsive pacing therapy. Olson suggests that accuracy may be increased by using multiple electrode pairs, but that would severely increase the cost of equipment and complexity of surgery required. An additional disadvantage of Olson is that it is difficult to sense the impedance of the heart cavity during the period immediately following the delivery of pacing pulses.
Each of the aforementioned hemodynamic indicators may have certain drawbacks associated with it. One drawback is that some hemodynamic indicators, such as QT time interval, may not respond rapidly enough to optimally control cardiac pacing. Another drawback is that the measurement of these indicators may require the use of sensors that must be delivered to locations that normally do not receive electrical stimulation. Therefore, additional leads may be required which undesirably add cost to the system and complexity to the surgical procedure during which the leads are implanted. It would be desirable if the pacemaker could use physiological parameters representative of the performance of the heart during each cardiac contraction to determine a heart rate that meets the patient's current level of metabolic need.
A known rate-responsive pacemaker which also optimizes cardiac performance at rest is described in U.S. Pat. No. 5,024,222 of Thacker, which is hereby incorporated by reference in its entirety. Thacker describes a rate-responsive pacemaker which senses the physiological needs of the patient's heart using an oxygen saturation (SO.sub.2) sensor and controls the pacing rate accordingly. When the patient is at rest, Thacker adjusts the pacing rate to a sub-optimal level and then adjusts the AV interval until hemodynamic performance is optimized to provide the lowest possible stimulation rate while retaining optimum cardiac performance.
Although many of the above-described hemodynamic indicators have been used in adjusting the operating parameters of the pacemaker, the physician typically sets the pacing parameters "open-loop" without knowing if the settings are hemodynamically optimal (a process where the patient returns to the physician's office periodically after the implantation for fine-tuning of the parameters). For example, while the base heart rate and AV interval are typically programmable by the physician in typical dual-chamber pacemakers, such implantable cardiac stimulating devices do not have the capability to "fine tune" these settings in a closed-loop fashion to achieve optimal cardiac performance. In fact, if the physician attempts to fine-tune the device to optimize hemodynamics, it must be performed open-loop.
In view of the deficiencies associated with the use of the IEGM or certain hemodynamic indicators, it would be desirable to provide an improved system which automatically determines the hemodynamically optimal pacemaker settings for a given patient. Ideally, such a system would use a signal that rapidly responds to sensed changes in the patient's condition (e.g., from rest to exercise), and is not subject to electrical interference from external sources or from pacemaker-induced after-potentials. During periods of activity, the system would provide an optimal heart rate. During rest, the system would provide hemodynamically optimal pacing by adjusting the base heart rate and the AV delay accordingly. Additionally, the system would provide optimal chronotropic stimulation (i.e., optimal timing of the delivered pacing pulses) to the patient by determining hemodynamically optimal pacing parameters and adjusting the device accordingly.